In many existing non-impact printers an image is built by repetitively producing lines of spots, each spot being formed by means of one of a number of recording sources forming a linear array of said recording sources. Each of said recording sources delivers a controlled amount of energy to a receptor surface in order to form a latent image on said receptor surface. Also, said non-impact printers comprise means for developing said latent images and transferring the developed images to the output medium, being paper or the like.
In such printers, multiple gradation levels of the optical density of each spot constituting an image to be printed are obtained by providing multiple energy levels for each spot, and as energy equals power times time, each of the recording sources is typically driven with a constant output power level, for a period of time proportional to the gradation value of the respective spot. This approach is much easier to implement than the approach wherein the multiple energy levels are obtained by providing multiple levels of output power for a constant period of time. In printers where the recording sources comprise LEDs for example, multiple optical energy levels are obtained by driving each of the LEDs by means of a source of constant current during a precisely controlled amount of time.
However, by applying the above mentioned techniques, the evenness of the optical density of the different spots of the printed image largely depends on the evenness of the energy levels supplied by the respective recording sources. It is now known that there are many causes of such unevenness of energy levels (the following list relates to LED-based printers, but similar problems are encountered with other non-impact printers). It is herein assumed that the recording sources are arranged in a plurality of recording modules, each recording module comprising a sub-set of recording sources so that the manufacturing of said linear array of recording sources becomes more feasible.
1) Due to manufacturing tolerances, the reference voltage or current which controls the average current level for each LED of a subset, can vary between different recording modules. PA1 2) The current supplied by said individual sources of constant current can vary due to manufacturing tolerances. PA1 3) Again due to manufacturing tolerances, but also due to the unequal ageing of the different LEDs, differences in the optical energy output of the LEDs can occur. PA1 4) The linear lens (e.g. an array of focusing fibres) used to project the light emitted by the LEDs typically shows a uneven attenuation over its length. PA1 5) Due to dirt, such as splinters produced when sawing the LED-chips, which can reside on the LED surface, the light emitted by the respective LED can become reduced. PA1 a linear array of recording sources for forming a multi-gradation latent image on a receptor surface, said linear array comprising a plurality of sub-sets of recording sources; PA1 means for developing said latent image into a visual image; PA1 means for transferring said visual image onto said medium; PA1 first correction storage means for storing first correction factors derived from measurements of the non-uniformity of the average energy output of the recording sources of said sub-sets of recording sources; PA1 first correction means for applying, for each sub-set of recording sources, said first correction factors; PA1 second correction storage means for storing second correction factors derived from said measurements of the non-uniformities of the energy output of each one of said individual recording sources, after said first correction factors have been applied; and PA1 second correction means for applying said second correction factors after applying said first correction factors. PA1 a linear array of recording sources for forming a multi-gradation latent image on a receptor surface, said linear array comprising a plurality of sub-sets of recording sources; PA1 means for developing said latent image into a visual image; PA1 means for transferring said visual image onto said medium; PA1 measuring the average energy output of each of said sub-sets of recording sources; PA1 generating and storing first correction factors derived from said measurements of the non-uniformity of the energy output of said sub-sets of recording sources; PA1 applying, for each sub-set of recording sources, said first correction factors; PA1 measuring the energy output of each individual one of said recording sources; PA1 generating and storing second correction factors derived from said measurements of the non-uniformity of the energy output of each individual one of said recording sources; and PA1 applying said second correction factors after applying said first correction factors. PA1 third correction storage means for storing third correction factors derived from measurement of the unevenness of the optical density of one or more printed images printed after the application of said first and second correction factors; and PA1 third correction means for providing a correction of the unevenness of the optical density of the image to be printed according to said stored third correction factors. PA1 a linear array of recording sources for forming a multi-gradation latent image on a receptor surface, said linear array comprising a plurality of sub-sets of recording sources; PA1 means for developing said latent image into a visual image; PA1 means for transferring said visual image onto said medium; PA1 means for storing correction factors derived from measurements of the non-uniformity of energy output of each individual one of said recording sources; and PA1 correction means for applying said correction factors,
Also, the means for rendering said latent image visual onto the target medium can show defects that introduce unevenness of the optical density of the produced images, said defects typically resulting in stripes or bands to become visible.
It is therefore desirable to provide for means to correct for the most substantial of these errors so that the unevenness in optical density of the produced images is brought to an acceptable level. This is especially true if said printers are, as opposed to binary printers wherein each spot can have only two possible gradation or density levels, capable of producing more than two density levels per spot.
In U.S. Pat. No. 4,855,760 (assigned to Fuji Photo film Co.) a method is disclosed for controlling the generation of multiple gradation levels for each spot, said method being applicable in a non-impact printer using a linear array of LEDs. Also described is a circuit for applying a correction to the gradation data fed to the LEDs in order to compensate for their non-uniform light-emitting intensity. Said correction is carried out by changing the gradation value of a spot to be printed so that the corrected gradation value compensates for the previously measured deviation of the intensity of the respective LED compared to the intensity of a reference LED. In order however to be able to correct for the LEDs having a too low energy output level, a number of gradation values are lost.
In U.S. Pat. No. 4,575,739 (assigned to Agfa-Gevaert N.V.) an apparatus is described incorporating means to control the average current of a subset of LEDs. Said apparatus provides means to select after measuring the average current of the LEDs of the subset, from an external array of four resistors, one of fifteen combinations of resistors that generates, from a fixed voltage, a reference current to which the current of each individual LED is made proportional. Apart from raising the production cost, this correction technique does not compensate for any differences in the individual current sources. Also, and this counts for most hard-wired correction techniques, it is difficult to repeat the correction process when, e.g. due to ageing, the non-uniformity pattern has changed.
In practice, it is found that the magnitude of the non-uniformities of the LEDs of an array can be substantial. Also, when said array of LEDs is being constructed by means of a number of recording modules containing a subset of LEDs, the magnitude of the non-uniformities of the average energy output of said subsets of LEDs add to the magnitude of the non-uniformities of the individual LEDs. Therefore, the number of gradation values needed for correcting weaker LEDs increases.
In addition, neither of the above techniques provides means to correct the unevenness of the optical density of printed images due to non-uniformities introduced by the process of developing a latent image, transferring said developed image onto a medium and fixing said transferred image onto the medium.
It is therefore an object of the invention to provide a printer incorporating a linear array of recording sources, which allows for correction of unevenness of the optical density of the produced image while reducing the number of gradation levels to be reserved for correction purposes and at the same time allows such correction to be easily repeated after said unevenness pattern has changed. A further object of the invention is to provide means to correct the unevenness of the optical density of an image introduced when such image is visualised from a latent image.